JPA的继承映射

JPA中的实体层次设计

这部分的内容基本与Hibernate一致.JPA同样支持3种类型的继承形式:

1.Single Table Strategy ,单表策略,一张表包含基类与子类的所有数据,很多情况下都是采用这样的冗余设计,通过一个discriminator来区分

2.Table Per Class Strategy ,每个子类对应一张表,每张表都拥有基类的属性

3.Join Strategy ,仍然是每个子类对应一张表，但此表中不包含基类的属性,仅仅是此子类的扩展属性,共享基类的属性

以一个例子来说明3种情况:

一.单表策略

比如Pet作为基类,Cat和Dog继承此类并拥有自己的扩展属性,如:

package com.denny_blue.ejb3.inheritance;

import java.io.Serializable;

import javax.persistence.DiscriminatorColumn;

import javax.persistence.DiscriminatorType;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.Inheritance;

import javax.persistence.InheritanceType;

@Entity

@Inheritance(strategy = InheritanceType.SINGLE_TABLE)

@DiscriminatorColumn(name = "animal_type", discriminatorType = DiscriminatorType.STRING)

public class Pet implements Serializable {

private int id;

private String name;

private double weight;

public Pet() {

}

@Id

@GeneratedValue(strategy = GenerationType.AUTO)

public int getId() {

return id;

}

public void setId(int id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public double getWeight() {

return weight;

}

public void setWeight(double weight) {

this.weight = weight;

}

}

Pet类值的注意的就是通过@Inheritance(strategy = InheritanceType.SINGLE_TABLE)确定采用单表策略,通过@DiscriminatorColumn确定了标志值的字段和类型，我想熟悉hibernate的朋友对这些都应该很熟悉.然后是两个子类:

//Cat.java

package com.denny_blue.ejb3.inheritance;

import javax.persistence.DiscriminatorColumn;

import javax.persistence.DiscriminatorType;

import javax.persistence.DiscriminatorValue;

import javax.persistence.Entity;

import javax.persistence.Inheritance;

import javax.persistence.InheritanceType;

@Entity

@Inheritance(strategy = InheritanceType.SINGLE_TABLE)

@DiscriminatorColumn(discriminatorType = DiscriminatorType.STRING)

@DiscriminatorValue("cat")

public class Cat extends Pet {

private String HairBall;

public String getHairBall() {

return HairBall;

}

public void setHairBall(String hairBall) {

HairBall = hairBall;

}

}

//Dog.java

package com.denny_blue.ejb3.inheritance;

import javax.persistence.DiscriminatorColumn;

import javax.persistence.DiscriminatorType;

import javax.persistence.DiscriminatorValue;

import javax.persistence.Entity;

import javax.persistence.Inheritance;

import javax.persistence.InheritanceType;

@Entity

@Inheritance(strategy=InheritanceType.SINGLE_TABLE)

@DiscriminatorColumn(discriminatorType=DiscriminatorType.STRING)

@DiscriminatorValue("dog")

public class Dog extends Pet {

private String trick;

public String getTrick() {

return trick;

}

public void setTrick(String trick) {

this.trick = trick;

}

}

两个子类最值的关注的就是@DiscriminatorValue注释,比如Cat的此值为cat,意味着当Cat类型的Entity存入数据库时,JPA将自动把cat的值赋给animal_type字段,Dog的值则为dog,由此就可以在同一张表中区分开两个不同的子类.

二.Table per Class

采用Table Per Class策略的话，每个子类都将单独建表，并且都独立拥有基类中的所有属性,互相之间不共享,在我们的例子中所要进行的修改很小,像这样:

//基类

@Entity

@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)

public class Pet implements Serializable {

private int id;

private String name;

private double weight;

........

//子类:不需要任何设置

@Entity

public class Dog extends Pet {

private String trick;

.......

.......

三.Join策略

每个子类同样独立建表,基类也独立建表，只不过所有的子类的表中只有扩展属性,他们共享基类的表,在我们的例子中修改下即可:

//基类

@Entity

@Inheritance(strategy = InheritanceType.JOINED)

public class Pet implements Serializable {

private int id;

private String name;

private double weight;

........

//子类

@Entity

@Inheritance(strategy = InheritanceType.JOINED)

public class Dog extends Pet {

private String trick;